Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
EARNINGS
Income (loss) from continuing operations		$ 789	$ 919	$ 946
Less: Net income (loss) attributable to noncontrolling interests		(1)	4	7
Income (loss) from continuing operations attributable to Key		790	915	939
Income (loss) from continuing operations attributable to Key common shareholders		753	892	917
Income (loss) from discontinued operations, net of taxes		1	1	(39)
Net income (loss) attributable to Key common shareholders		$ 754	$ 893	$ 878
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (in shares)		927,816	836,846	871,464
Effect of convertible preferred stock (in shares)		0	0	0
Effect of common share options and other stock awards (in shares)		10,720	7,643	6,735
Weighted-average common shares and potential common shares outstanding (000) (in shares)	[1]	938,536	844,489	878,199
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in dollars per share)		$ 0.81	$ 1.06	$ 1.05
Income (loss) from discontinued operations, net of taxes (in dollars per share)		0.00	0.00	(0.04)
Net income (loss) attributable to Key common shareholders (in dollars per share)	[2]	0.81	1.06	1.01
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in dollars per share)		0.80	1.05	1.04
Income (loss) from discontinued operations, net of taxes (in dollars per share)		0.00	0.00	(0.04)
Net income (loss) attributable to Key common shareholders (in dollars per share)	[2]	$ 0.80	$ 1.05	$ 0.99
Series A Preferred Stock [Member]
EARNINGS
Less: Dividends on Preferred Stock		$ 37	$ 23	$ 22

[1] Assumes conversion of common share options and other stock awards and/or convertible preferred stock, as applicable.

[2] EPS may not foot due to rounding.